Polyaxial pedicle screw

ABSTRACT

A polyaxial bone anchor including a housing, a bone screw, and a retainer for pivotably coupling the head of the bone screw to the housing. The retainer is positioned into the bore of the housing and includes a plurality of alternating tabs and slots circumferentially arranged to define a cavity for receiving the head portion of the bone screw therein. The retainer is axially moveable in the housing from a first position in which the head portion is not passable through the lower opening of the retainer to a second position in which the head portion is passable through the lower opening of the retainer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/US2011/049533, filed on Aug.29, 2011, which claims priority to U.S. Provisional Application61/378,182, filed on Aug. 30, 2010, the entire disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The disclosure is directed to vertebral anchors for use with orthopedicfixation systems. More particularly, the disclosure is directed topolyaxial pedicle screws including structures for securely coupling abone screw to a housing of the polyaxial pedicle screw.

BACKGROUND

The spinal column of a patient includes a plurality of vertebrae linkedto one another by facet joints and an intervertebral disc locatedbetween adjacent vertebrae. The facet joints and intervertebral discallow one vertebra to move relative to an adjacent vertebra, providingthe spinal column a range of motion. Diseased, degenerated, damaged, orotherwise impaired facet joints and/or intervertebral discs may causethe patient to experience pain or discomfort and/or loss of motion, thusprompting surgery to alleviate the pain and/or restore motion of thespinal column.

One possible method of treating these conditions is to immobilize aportion of the spine to allow treatment. Traditionally, immobilizationhas been accomplished by rigid stabilization. For example, in aconventional spinal fusion procedure, a surgeon restores the alignmentof the spine or the disc space between vertebrae by installing a rigidfixation rod between pedicle screws secured to adjacent vertebrae. Bonegraft is placed between the vertebrae, and the fixation rod cooperateswith the screws to immobilize the two vertebrae relative to each otherso that the bone graft may fuse with the vertebrae.

Dynamic stabilization has also been used in spinal treatment procedures.Dynamic stabilization does not result in complete immobilization, butinstead permits a degree of mobility of the spine while also providingsufficient support and stabilization to effect treatment. One example ofa dynamic stabilization system is the Dynesys® system available fromZimmer Spine, Inc. of Minneapolis, Minn. Such dynamic stabilizationsystems typically include a flexible member positioned between pediclescrews installed in adjacent vertebrae of the spine. A flexible cord canbe threaded through the bore in the flexible member and secured to thepedicle screws while cooperating with the flexible member to permitmobility of the spine.

Thus, it may be desirable to provide alternative vertebral anchors whichmay be used in spinal stabilization systems which are configured tosecure elongate members or other structures to one or more vertebrae ofa spinal segment of a spinal column in a desired configuration intreating various spinal disorders.

Accordingly, it is desirable to develop a pedicle screw that providespolyaxial rotation which is easily assembled and configured to besecured in a desired angular orientation when secured to an elongatedmember of a vertebral stabilization system.

SUMMARY

The disclosure is directed to several alternative designs, materials andmethods of assembling vertebral anchor structures and assemblies.

Accordingly, one illustrative embodiment is a polyaxial bone anchor. Thepolyaxial bone anchor includes a housing having an upper end, a lowerend and a bore extending through the housing from the upper end to thelower end. The bore opens out at the lower end at a lower opening. Thehousing also includes a channel configured for receiving an elongatestabilization member therethrough which extends from a first sidesurface of the housing to a second side surface of the housing oppositethe first side surface transverse to the bore. The bone anchor alsoincludes a retainer positioned into the bore of the housing which has anoutermost diameter greater than a diameter of the lower opening. Theretainer includes a plurality of alternating tabs and slotscircumferentially arranged to define a cavity therein. Also included isa bone screw including a head portion and a shank extending from thehead portion. The head portion of the bone screw is positionable in thecavity of the retainer with the shank extending from the lower end ofthe housing by deflecting the tabs radially outward to enlarge a loweropening of the retainer into the cavity from a diameter less than adiameter of the head portion to a diameter greater than or equal to thediameter of the head portion. The bone anchor also includes means forapplying a force to move the retainer toward the lower end of thehousing while the retainer is positioned in the bore.

Another illustrative embodiment is a polyaxial bone anchor. The boneanchor includes a housing having an upper end, a lower end and a boreextending through the housing from the upper end to the lower end. Thebore opens out at the lower end at a lower opening. The housing alsoincludes a channel configured for receiving an elongate stabilizationmember therethrough which extends from a first side surface of thehousing to a second side surface of the housing opposite the first sidesurface transverse to the bore. The bone anchor also includes a retainerpositionable in the bore of the housing. The retainer is movable in thebore of the housing between a first position and a second position. Theretainer is closer to the lower end of the housing in the first positionand closer to the upper end of the housing in the second position.Additionally, the bone anchor includes a bone screw including a headportion and a shank extending from the head portion. The head portion ofthe bone screw is positionable in a cavity of the retainer with theshank extending from the lower end of the housing. The head portion ofthe bone screw is insertable into the cavity of the retainer from thelower end of the housing when the retainer is in the second position,but is not removable from the cavity of the retainer when the retaineris in the first position.

Another illustrative embodiment is a polyaxial bone anchor including ahousing, a bone screw, a spacer, a collar and a resilient spring means.The housing has an upper end, a lower end and a bore extending throughthe housing from the upper end to the lower end. A lower portion of thehousing includes a plurality of deflectable tabs arranged around aperimeter of the housing. The housing includes a channel configured forreceiving an elongate stabilization member therethrough which extendsfrom a first side surface of the housing to a second side surface of thehousing opposite the first side surface transverse to the bore. Thespacer is positionable in the bore of the housing and movable in thebore of the housing between a first position and a second position. Thespacer is closer to the lower end of the housing in the first positionand closer to the upper end of the housing in the second position. Thebone screw includes a head portion and a shank extending from the headportion. The head portion of the bone screw is positionable in a cavityof the spacer with the shank extending from the lower end of thehousing. The collar is positionable circumferentially exterior of theplurality of deflectable tabs to inhibit outward radial deflection ofthe plurality of tabs. The resilient spring means biases the spacertoward the first position into engagement with the head portion of thebone screw. The head portion of the bone screw is insertable into thebore of the housing from the lower end of the housing by deflecting theplurality of tabs radially outward, but is not removable from thehousing when the collar is positioned circumferentially exterior of theplurality of deflectable tabs.

Yet another illustrative embodiment is a method of assembling apolyaxial bone anchor. A housing is provided having an upper end, alower end and a bore extending through the housing from the upper end tothe lower end. The bore opens out at the lower end at a lower opening.The housing also includes a channel configured for receiving an elongatestabilization member therethrough which extends from a first sidesurface of the housing to a second side surface of the housing oppositethe first side surface transverse to the bore. A retainer is insertedinto the bore of the housing from the lower end of the housing bypassing the retainer through the lower opening. The retainer has anoutermost diameter greater than a diameter of the lower opening. Theretainer includes a plurality of alternating tabs and slots formedtherein providing the retainer with sufficient flexibility to be urgedthrough the lower opening from the lower end of the housing by radiallycompressing the retainer. A head portion of a bone screw is insertedinto a cavity of the retainer from the lower end of the housing bymoving the retainer toward the upper end of the housing allowing theplurality of tabs to splay radially outward to accommodate passage ofthe head portion of the bone screw into the cavity of the retainer. Theretainer, with the head portion of the bone screw positioned in thecavity of the retainer, is urged back toward the lower end of thehousing to retain the head portion of the bone screw in the cavity ofthe retainer. The retainer is biased toward the lower end of the housingby a biasing spring means.

The above summary of some example embodiments is not intended todescribe each disclosed embodiment or every implementation of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments in connection withthe accompanying drawings, in which:

FIG. 1 is an exploded perspective view of components of an exemplaryvertebral anchor:

FIG. 1A is a cross-sectional view of the vertebral anchor of FIG. 1 withthe housing coupled to the head portion of the bone screw:

FIG. 1B is a cross-sectional view of the vertebral anchor of FIG. 1while coupling the housing to the head portion of the bone screw;

FIG. 2 is an exploded perspective view of components of anotherexemplary vertebral anchor;

FIG. 2A is a cross-sectional view of the vertebral anchor of FIG. 2 withthe housing coupled to the head portion of the bone screw;

FIG. 2B is a cross-sectional view of the vertebral anchor of FIG. 2,transverse to the cross-sectional view of FIG. 2A, with the housingcoupled to the head portion of the bone screw;

FIG. 2C is a cross-sectional view of the vertebral anchor of FIG. 2while coupling the housing to the head portion of the bone screw;

FIG. 3 is an exploded perspective view of components of anotherexemplary vertebral anchor;

FIG. 3A is a cross-sectional view of the vertebral anchor of FIG. 3 withthe housing coupled to the head portion of the bone screw;

FIG. 3B is a cross-sectional view of the vertebral anchor of FIG. 3while coupling the housing to the head portion of the bone screw;

FIG. 4 is an exploded perspective view of components of anotherexemplary vertebral anchor;

FIG. 4A is a cross-sectional view of the vertebral anchor of FIG. 4 withthe housing coupled to the head portion of the bone screw:

FIG. 4B is a cross-sectional view of the vertebral anchor of FIG. 4while coupling the housing to the head portion of the bone screw;

FIG. 5 is an exploded perspective view of components of yet anotherexemplary vertebral anchor;

FIG. 5A is a cross-sectional view of the vertebral anchor of FIG. 5 withthe housing coupled to the head portion of the bone screw; and

FIG. 5B is a cross-sectional view of the vertebral anchor of FIG. 5while coupling the housing to the head portion of the bone screw.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit aspects of the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

For the following defined terms, these definitions shall be applied,unless a different definition is given in the claims or elsewhere inthis specification.

All numeric values are herein assumed to be modified by the term“about”, whether or not explicitly indicated. The term “about” generallyrefers to a range of numbers that one of skill in the art would considerequivalent to the recited value (i.e., having the same function orresult). In many instances, the term “about” may be indicative asincluding numbers that are rounded to the nearest significant figure.

The recitation of numerical ranges by endpoints includes all numberswithin that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4,and 5).

Although some suitable dimensions, ranges and/or values pertaining tovarious components, features and/or specifications are disclosed, one ofskill in the art, incited by the present disclosure, would understanddesired dimensions, ranges and/or values may deviate from thoseexpressly disclosed.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. As used in this specification and theappended claims, the term “or” is generally employed in its senseincluding “and/or” unless the content clearly dictates otherwise.

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The detailed description and the drawings, which are notnecessarily to scale, depict illustrative embodiments and are notintended to limit the scope of the invention. The illustrativeembodiments depicted are intended only as exemplary. Selected featuresof any illustrative embodiment may be incorporated into an additionalembodiment unless clearly stated to the contrary.

Referring now to FIG. 1, there is shown a first exemplary embodiment ofa vertebral anchor 10, shown as a polyaxial pedicle screw. The vertebralanchor 10 may include several components. For example, the vertebralanchor 10 may include a housing 12 pivotably coupled to a bone screw 14.The bone screw 14 may include a shaft portion 16, which may in someinstances be threaded, extending from a head portion 18, which may insome instances be spherically shaped. The shaft 16 may be configured tobe installed into a bony region of a vertebra of the spinal column. Forexample, the shaft 16 may be installed into a pedicle of a vertebra, orother region of a vertebra. The bone screw 14 may be pivotable relativeto the housing 12 such that the longitudinal axis of the bone screw 14is positioned at one of multiple angular orientations relative to thelongitudinal axis of the housing 12.

The housing 12 may include a channel 20, such as a U-shaped channelextending from one side of the housing 12 to an opposite second side ofthe housing 12. The channel 20 may be defined between opposing first andsecond legs 22, 24 of the housing 12. The housing 12 may also include abore 26 extending through the housing 12 along a longitudinal axis fromthe upper end 28 to the lower end 30 of the housing 12 which intersectsthe channel 20.

The housing 12 of the vertebral anchor 10 may be configured to receivean elongate member 40 of a vertebral stabilization system, such as arigid or flexible fixation element, including a spinal rod or flexiblecord, therein. For example, the channel 20 may be open to the upper end28 of the housing 12 such that the elongate member 40 may be positionedin the channel 20 in a top-loaded fashion in which the elongate member40 is moved into the channel 20 of the housing 12 in a directiongenerally perpendicular to the longitudinal axis of the channel 20 ofthe housing 12.

The vertebral anchor 10 may also include a securing element, such as athreaded fastener 34 (e.g., a set screw, cap) configured to rotatablyengage the housing 12 to secure a portion of the elongate member 40 inthe channel 20. For example, the threaded fastener 34 may includethreads which mate with a threaded portion 32 formed in the legs 22, 24of the housing 12. In other embodiments, the fastener 34 may include oneor more flanges, cam surfaces, or other engagement features that engagewith one or more channels, grooves, surfaces, or other engagementfeatures of the housing 12 through rotation of the fastener 34. Thefastener 34 may be rotatably engaged between the spaced apart legs 22,24 of the housing 12 which define the channel 20 therebetween.

The vertebral anchor 10 may also include one or more components forcoupling the housing 12 to the head portion 18 of the bone screw 14. Forinstance, the vertebral anchor 10 may include a retainer 42 positionablein the bore 26 of the housing 12 which includes a cavity 44 forreceiving the head portion 18 of the bone screw 14 therein. In someinstances, the cavity 44 may be a spherically concave cavitycomplementing the spherical shape of the head portion 18 of the bonescrew 14. The retainer 42 may be formed of a resilient material, such asa pliable polymeric material or a malleable metallic material, providingthe retainer 42 a desired amount of flexibility. The retainer 42 mayalso include a plurality of alternating tabs 46 and slots 48 spacedaround a periphery of the retainer 42 enhancing the flexibility of theretainer 42. For example, a radially inward force may be exerted on thetabs 46 to deflect the tabs 46 radially inward to radially compress theretainer 42, whereas a radially outward force may be exerted on the tabs46 to deflect or splay the tabs 46 radially outward to radially enlargethe lower opening into the cavity 44 of the retainer 42.

The retainer 42 illustrated in FIG. 1 includes a first subset of slots48 a opening out to the upper end of the retainer 42 and a second subsetof slots 48 b opening out to the lower end of the retainer 42. The slots48 a may alternate with the slots 48 b around the circumference of theretainer 42.

The vertebral anchor 10 may also include a spacer 50 extending from theretainer 42 toward the upper end 28 of the housing 12. The spacer 50 maybe axially movable relative to the retainer 42. The spacer 50 mayinclude a first portion extending from the retainer 42 which isconfigured to engage an elongate stabilization member 40 disposed in thechannel 20 of the housing 12 and a second portion extending into a boreof the retainer 42 which is configured to engage the head portion 18 ofthe bone screw 14. The spacer 50 may include an enlarged annular portionpositioned in the bore of the retainer 42 which interlocks with aradially inward extending annular lip of the retainer 42.

The vertebral anchor 10 may further include a resilient spring meansbiasing the retainer 42 toward the lower end 30 of the housing 12. Asshown in FIG. 1, the resilient spring means may be a wave washer 52,however, in other instances the resilient spring means may be a helicalspring, elastomeric member, an integral portion of the retainer 42, oranother structure configured to urge the retainer 42 toward the lowerend 30 of the housing 12.

The arrangement of components for coupling the housing 12 to the headportion 18 of the bone screw 14 is further illustrated in FIG. 1A. Asshown in FIG. 1A, the retainer 42 may be positioned in an enlargedportion 36 of the bore 26, surrounding the head portion 18 of the bonescrew 14. The spacer 50 may be positioned between the retainer 42 andthe elongate member 40, with a portion of the spacer 50 extending intothe bore of the retainer 42 and directly engaging the head portion 18 ofthe bone screw 14. The wave washer 52 may be positioned in the bore 26of the housing 12 and compressed between an annular rim of the housing12 facing the lower end 30 of the housing 12 and an annular surface ofthe retainer 42 and/or spacer 50 facing the upper end 28 of the housing12.

The retainer 42 may be movable in the bore 26 of the housing 12 alongthe longitudinal axis of the bore 26 between a first position in whichthe retainer 42 is closer to the lower end 30 of the housing 12 and asecond position in which the retainer 42 is closer to the upper end 28of the housing 12. The wave washer 52, or other resilient biasing means,may bias the retainer 42 toward the first position until a sufficientforce is applied to the retainer 42 to overcome the biasing force of thewave washer 52 and moves the retainer 42 to the second position.

The retainer 42 may have an outermost diameter which is greater than thediameter of the lower opening 38 of the bore 26 extending through thehousing 12, yet the outermost diameter of the retainer 42 may be lessthan an enlarged portion 36 of the bore 26 in which the retainer 42 ispositioned, providing an annular space 54 between the outercircumferential surface of the retainer 42 and the circumferentialsurface of the bore 26. In some instances, the housing 12 may include anannular rim 56 defining the lower opening 38, in which the diameter ofthe lower opening 38 at the annular rim 56 is less than a diameter ofthe enlarged portion 36 of the bore 26 of the housing 12 toward theupper end 28 of the housing 12 from the annular rim 56. When in thefirst position, the wave washer 52 may push the retainer 42 against theannular rim 56, preventing the retainer 42 from radially expanding.

During assembly of the vertebral anchor 10, the retainer 42, as well asthe wave washer 52 and the spacer 50 may be inserted into the loweropening 38 of the housing 12. For example, the plurality of alternatingtabs 46 and slots 48 formed around the circumference of the retainer 42may provide the retainer 42 with sufficient flexibility to be urgedthrough the lower opening 38 from the lower end 30 of the housing 12 byradially compressing the retainer 42.

With the retainer 42, resilient spring means (e.g., the wave washer 52)and other components positioned in the bore 26 of the housing 12, thehead portion 18 of the bone screw 14 may be inserted into the cavity 44of the retainer 42 through the lower opening 38 from the lower end 30 ina bottom loaded manner. The diameter of the head portion 18 of the bonescrew 14 may be less than the diameter of the lower opening 38 at theannular rim 56 to allow the head portion 18 to pass therethrough. Asshown in FIG. 1B, the head portion 18 of the bone screw 14, or anotherstructure, may apply a force against the retainer 42 opposing andovercoming the biasing force of the wave washer 52 which urges theretainer 42 to the second position in which the retainer 42 is movedtoward the upper end 28 of the housing 12 along the longitudinal axis ofthe bore 26. Now positioned in an enlarged diameter portion 36 of thebore 26 and radially unconstrained by the interior surface of the bore26 and/or the annular rim 56 at the lower opening 38 of the housing 12,the flexibility of the retainer 42 allows the retainer 42 to be radiallyexpanded. For example, the plurality of tabs 46 of the retainer 42 maybe deflected radially outward in order to allow the head portion 18 ofthe bone screw 14 to pass into the cavity 44 of the retainer 42. Thepresence of the annular space 54 allows the retainer 42 to radiallyexpand to accommodate insertion of the head portion 18 into the cavity44.

Once the head portion 18 of the bone screw 14 is positioned in thecavity 44, the applied force to the retainer 42 may be removed, allowingthe biasing force of the wave washer 52 or other biasing means to movethe retainer 42 back to the first position toward the lower end 30 ofthe housing 12 and into engagement with the reduced diameter annularportion of the housing 12 to prevent further radial expansion orsplaying of the retainer 42. In some instances, the retainer 42 mayinclude a lower beveled surface which contacts the annular rim 56 of thehousing 12 to urge the tabs 46 of the retainer 42 radially inward and/orprevent radial splaying to secure the head portion 18 of the bone screw14 in the cavity 44 of the retainer 42.

When an elongate member 40 is secured in the channel 20 of the housing12, a clamping force may be exerted against the head portion 18 of thebone screw 14. However, because the lower opening of the retainer 42when at the first position has a diameter less than the diameter of thehead portion 18 of the bone screw 14, the head portion 18 is preventedfrom being removed from the cavity 44 of the retainer 42 since theannular rim 56 of the housing 12 resists radial expansion of the loweropening of the retainer 42 when pressed thereagainst.

Another exemplary embodiment of a vertebral anchor 110, shown as apolyaxial pedicle screw, is illustrated in FIG. 2. The vertebral anchor110 may include several components. For example, the vertebral anchor110 may include a housing 112 pivotably coupled to a bone screw 114. Thebone screw 114 may include a shaft portion 116, which may in someinstances be threaded, extending from a head portion 118, which may insome instances be spherically shaped. The shaft 116 may be configured tobe installed into a bony region of a vertebra of the spinal column. Forexample, the shaft 116 may be installed into a pedicle of a vertebra, orother region of a vertebra. The bone screw 114 may be pivotable relativeto the housing 112 such that the longitudinal axis of the bone screw 114is positioned at one of multiple angular orientations relative to thelongitudinal axis of the housing 112.

The housing 112 may include a channel 120, such as a U-shaped channelextending from one side of the housing 112 to an opposite second side ofthe housing 112. The channel 120 may be defined between opposing firstand second legs 122, 124 of the housing 112. The housing 112 may alsoinclude a bore 126 extending through the housing 112 along alongitudinal axis from the upper end 128 to the lower end 130 of thehousing 112 which intersects the channel 120.

The housing 112 of the vertebral anchor 10 may be configured to receivean elongate member 140 of a vertebral stabilization system, such as arigid or flexible fixation element, including a spinal rod or flexiblecord, therein. For example, the channel 120 may be open to the upper end128 of the housing 112 such that the elongate member 140 may bepositioned in the channel 120 in a top-loaded fashion in which theelongate member 140 is moved into the channel 120 of the housing 112 ina direction generally perpendicular to the longitudinal axis of thechannel 120 of the housing 112.

The vertebral anchor 110 may also include a securing element, such as athreaded fastener 134 (e.g., a set screw, cap) configured to rotatablyengage the housing 112 to secure a portion of the elongate member 140 inthe channel 120. For example, the threaded fastener 134 may includethreads which mate with a threaded portion 132 formed in the legs 122,124 of the housing 112. In other embodiments, the fastener 134 mayinclude one or more flanges, cam surfaces, or other engagement featuresthat engage with one or more channels, grooves, surfaces, or otherengagement features of the housing 112 through rotation of the fastener134. The fastener 134 may be rotatably engaged between the spaced apartlegs 122, 124 of the housing 112 which define the channel 120therebetween.

The vertebral anchor 110 may also include one or more components forcoupling the housing 112 to the head portion 118 of the bone screw 114.For instance, the vertebral anchor 110 may include a retainer 142positionable in the bore 126 of the housing 112 which includes a cavity144 for receiving the head portion 118 of the bone screw 114 therein. Insome instances, the cavity 144 may be a spherically concave cavitycomplementing the spherical shape of the head portion 118 of the bonescrew 114. The retainer 142 may be formed of a resilient material, suchas a pliable polymeric material or a malleable metallic material,providing the retainer 142 a desired amount of flexibility. A lowerportion of the retainer 142 may also include a plurality of alternatingtabs 146 and slots 148 spaced around a periphery of the lower portion ofthe retainer 142 enhancing the flexibility of the lower portion of theretainer 142. For example, a radially inward force may be exerted on thetabs 146 to deflect the tabs 146 radially inward to radially compressthe retainer 142, whereas a radially outward force may be exerted on thetabs 146 to deflect or splay the tabs 146 radially outward to radiallyenlarge the lower opening into the cavity 144 of the retainer 142.

The upper portion of the retainer 142 illustrated in FIG. 2 may furtherinclude first and second legs 158, 160 defining a channel 162therebetween aligned with the channel 120 of the housing 112 forreceiving an elongate stabilization member 140 therethrough.

The vertebral anchor 110 may also include a spacer 150 extending fromthe retainer 142 toward the upper end 128 of the housing 112. The spacer150 may be axially movable relative to the retainer 142. The spacer 150may include a first portion extending from the retainer 142 which isconfigured to engage an elongate stabilization member 140 disposed inthe channel 120 of the housing 112 and a second portion extending into abore of the retainer 142 which is configured to engage the head portion118 of the bone screw 114. In some instances, the spacer 150 may includestructure, such as an enlarged annular portion which interlocks withstructure of the retainer 142, such as an annular lip of the retainer142.

In some instances, the vertebral anchor 110 may further include aresilient spring means (not shown) biasing the retainer 142 toward thelower end 130 of the housing 112. In some instances, the resilientspring means may be a wave washer, a helical spring, elastomeric member,an integral portion of the retainer 142, or another structure configuredto urge the retainer 142 toward the lower end 130 of the housing 112.

The arrangement of components for coupling the housing 112 to the headportion 118 of the bone screw 114 is further illustrated in FIGS. 2A and2B. As shown, the lower portion of the retainer 142 may be positioned inan enlarged portion 136 of the bore 126, surrounding the head portion118 of the bone screw 114. The legs 158, 160 of the retainer 142 may bealigned with the legs 122, 124 of the housing 112 such that the channel162 defined between the legs 158, 160 is aligned with the channel 120defined between the legs 122, 124 of the housing 112. The spacer 150 maybe positioned between the retainer 142 and the elongate member 140, witha portion of the spacer 150 extending into the bore of the retainer 142and directly engaging the head portion 118 of the bone screw 114.

The retainer 142 may be movable in the bore 126 of the housing 112 alongthe longitudinal axis of the bore 126 between a first position in whichthe retainer 142 is closer to the lower end 130 of the housing 112 and asecond position in which the retainer 142 is closer to the upper end 128of the housing 112. In some instances, a resilient biasing means, suchas a wave washer, a helical spring, elastomeric member, an integralportion of the retainer 142, or another structure, may bias the retainer142 toward the first position until a sufficient force is applied to theretainer 142 to overcome the biasing force of the wave washer 152 andmoves the retainer 142 to the second position.

The retainer 142 may have an outermost diameter which is greater thanthe diameter of the lower opening 138 of the bore 126 extending throughthe housing 112, yet the outermost diameter of the retainer 142 may beless than an enlarged portion 136 of the bore 126 in which the retainer142 is positioned, providing an annular space 154 between the outercircumferential surface of the retainer 142 and the circumferentialsurface of the bore 126. In some instances, the housing 112 may includean annular rim 156 defining the lower opening 138, in which the diameterof the lower opening 138 at the annular rim 156 is less than a diameterof the enlarged portion 136 of the bore 126 of the housing 112 towardthe upper end 128 of the housing 112 from the annular rim 156. When inthe first position, a resilient biasing member may push the retainer 142against the annular rim 156, preventing the retainer 142 from radiallyexpanding. Alternatively, a clamping force exerted against the elongatemember 140 by the fastener 134 may exert a force through the spacer 150to the retainer 142, push the retainer 142 against the annular rim 156.

During assembly of the vertebral anchor 110, the retainer 142 and thespacer 150 may be inserted into the bore 126 of the housing 112 throughthe lower opening 138 of the housing 112 or from the upper end 128 ofthe housing 112. For example, the plurality of alternating tabs 146 andslots 148 formed around the circumference of the retainer 142 mayprovide the retainer 142 with sufficient flexibility to be radiallycompressed when being inserted into the bore 126.

With the retainer 142, spacer 150, and resilient spring means ifpresent, positioned in the bore 126 of the housing 112, the head portion118 of the bone screw 114 may be inserted into the cavity 144 of theretainer 142 through the lower opening 138 from the lower end 130 in abottom loaded manner. The diameter of the head portion 118 of the bonescrew 114 may be less than the diameter of the lower opening 138 at theannular rim 156 to allow the head portion 118 to pass therethrough. Asshown in FIG. 2C, the head portion 118 of the bone screw 114, or anotherstructure, may apply a force against the retainer 142 opposing andovercoming any biasing force, and thus urging the retainer 142 to thesecond position in which the retainer 142 is moved toward the upper end128 of the housing 112 along the longitudinal axis of the bore 126. Nowpositioned in an enlarged diameter portion 136 of the bore 126 andradially unconstrained by the interior surface of the bore 126 and/orthe annular rim 156 at the lower opening 138 of the housing 112, theflexibility of the retainer 142 allows the lower portion of the retainer142 to be radially expanded. For example, the plurality of tabs 146 ofthe retainer 142 may be deflected radially outward in order to allow thehead portion 118 of the bone screw 114 to pass into the cavity 144 ofthe retainer 142. The presence of the annular space 154 allows theretainer 142 to radially expand to accommodate insertion of the headportion 118 into the cavity 144.

Once the head portion 118 of the bone screw 114 is positioned in thecavity 144, the applied force to the retainer 142 may be removed,allowing the retainer 142 to move back to the first position toward thelower end 130 of the housing 112 and into engagement with the reduceddiameter annular portion of the housing 112 to prevent further radialexpansion or splaying of the lower portion of the retainer 142. In someinstances, the retainer 142 may include a lower beveled surface whichcontacts the annular rim 156 of the housing 112 to urge the tabs 146 ofthe retainer 142 radially inward and/or prevent radial splaying tosecure the head portion 118 of the bone screw 114 in the cavity 144 ofthe retainer 142.

When an elongate member 140 is secured in the channel 120 of the housing112, a clamping force may be exerted against the head portion 118 of thebone screw 114. However, because the lower opening of the retainer 142when at the first position has a diameter less than the diameter of thehead portion 118 of the bone screw 114, the head portion 118 isprevented from being removed from the cavity 144 of the retainer 142since the annular rim 156 of the housing 112 resists radial expansion ofthe lower opening of the retainer 142 when pressed thereagainst.

Another exemplary embodiment of a vertebral anchor 210, shown as apolyaxial pedicle screw, is illustrated in FIG. 3. The vertebral anchor210 may include several components. For example, the vertebral anchor210 may include a housing 212 pivotably coupled to a bone screw 214. Thebone screw 214 may include a shaft portion 216, which may in someinstances be threaded, extending from a head portion 218, which may insome instances be spherically shaped. The shaft 216 may be configured tobe installed into a bony region of a vertebra of the spinal column. Forexample, the shaft 216 may be installed into a pedicle of a vertebra, orother region of a vertebra. The bone screw 214 may be pivotable relativeto the housing 212 such that the longitudinal axis of the bone screw 214is positioned at one of multiple angular orientations relative to thelongitudinal axis of the housing 212.

The housing 212 may include a channel 220, such as a U-shaped channelextending from one side of the housing 212 to an opposite second side ofthe housing 212. The channel 220 may be defined between opposing firstand second legs 222, 224 of the housing 212. The housing 212 may alsoinclude a bore 226 extending through the housing 212 along alongitudinal axis from the upper end 228 to the lower end 230 of thehousing 212 which intersects the channel 220.

The housing 212 of the vertebral anchor 210 may be configured to receivean elongate member 240 of a vertebral stabilization system, such as arigid or flexible fixation element, including a spinal rod or flexiblecord, therein. For example, the channel 220 may be open to the upper end228 of the housing 212 such that the elongate member 240 may bepositioned in the channel 220 in a top-loaded fashion in which theelongate member 240 is moved into the channel 220 of the housing 212 ina direction generally perpendicular to the longitudinal axis of thechannel 220 of the housing 212.

The vertebral anchor 210 may also include a securing element, such as athreaded fastener (not shown) configured to rotatably engage the housing212 to secure a portion of the elongate member 240 in the channel 220.For example, the threaded fastener may include threads which mate with athreaded portion 232 formed in the legs 222, 224 of the housing 212. Inother embodiments, the fastener may include one or more flanges, camsurfaces, or other engagement features that engage with one or morechannels, grooves, surfaces, or other engagement features of the housing212 through rotation of the fastener. The fastener may be rotatablyengaged between the spaced apart legs 222, 224 of the housing 212 whichdefine the channel 220 therebetween.

The vertebral anchor 210 may also include one or more components forcoupling the housing 212 to the head portion 218 of the bone screw 214.For instance, the vertebral anchor 210 may include a retainer 242positionable in the bore 226 of the housing 212 which includes a cavity244 for receiving the head portion 218 of the bone screw 214 therein. Insome instances, the cavity 244 may be a spherically concave cavitycomplementing the spherical shape of the head portion 218 of the bonescrew 214. The retainer 242 may be formed of a resilient material, suchas a pliable polymeric material or a malleable metallic material,providing the retainer 242 a desired amount of flexibility. The retainer242 may also include a plurality of alternating tabs 246 and slots 248spaced around a periphery of the retainer 242 enhancing the flexibilityof the retainer 242. For example, a radially inward force may be exertedon the tabs 246 to deflect the tabs 246 radially inward to radiallycompress the retainer 242, whereas a radially outward force may beexerted on the tabs 246 to deflect or splay the tabs 246 radiallyoutward to radially enlarge the lower opening into the cavity 244 of theretainer 242.

The vertebral anchor 210 may further include a resilient spring meansbiasing the retainer 242 toward the lower end 230 of the housing 212. Asshown in FIG. 3, the resilient spring means may be a wave washer 252,however, in other instances the resilient spring means may be a helicalspring, elastomeric member, an integral portion of the retainer 242, oranother structure configured to urge the retainer 242 toward the lowerend 230 of the housing 212.

The arrangement of components for coupling the housing 212 to the headportion 218 of the bone screw 214 is further illustrated in FIG. 3A. Asshown in FIG. 3A, the retainer 242 may be positioned in an enlargedportion 236 of the bore 226, surrounding the head portion 218 of the 20bone screw 214. The wave washer 252 may be positioned in the bore 226 ofthe housing 212 and compressed between an annular rim of the housing 212facing the lower end 230 of the housing 212 and an annular surface ofthe retainer 242 facing the upper end 228 of the housing 212. An upperportion of the retainer 242 may extend through the wave washer 252 todirectly engage the elongate member 240.

The retainer 242 may be movable in the bore 226 of the housing 212 alongthe longitudinal axis of the bore 226 between a first position in whichthe retainer 242 is closer to the lower end 230 of the housing 212 and asecond position in which the retainer 242 is closer to the upper end 228of the housing 212. The wave washer 252, or other resilient biasingmeans, may bias the retainer 242 toward the first position until asufficient force is applied to the retainer 242 to overcome the biasingforce of the wave washer 252 and moves the retainer 242 to the secondposition.

The retainer 242 may have an outermost diameter which is greater thanthe diameter of the lower opening 238 of the bore 226 extending throughthe housing 212, yet the outermost diameter of the retainer 242 may beless than an enlarged portion 236 of the bore 226 in which the retainer242 is positioned, providing an annular space 254 between the outercircumferential 5 surface of the retainer 242 and the circumferentialsurface of the bore 226. In some instances, the housing 212 may includean annular rim 256 defining the lower opening 238, in which the diameterof the lower opening 238 at the annular rim 256 is less than a diameterof the enlarged portion 236 of the bore 226 of the housing 212 towardthe upper end 228 of the housing 212 from the annular rim 256. When inthe first position, the wave washer 252 may push the retainer 242against the annular rim 256, preventing the retainer 242 from radiallyexpanding.

During assembly of the vertebral anchor 210, the retainer 242, as wellas the wave washer 252, may be inserted into the lower opening 238 ofthe housing 212. For example, the plurality of alternating tabs 246 andslots 248 formed around the circumference of the retainer 242 mayprovide the retainer 242 with sufficient flexibility to be urged throughthe lower opening 238 from the lower end 230 of the housing 212 byradially compressing the retainer 242.

With the retainer 242, resilient spring means (e.g., the wave washer252) and any other components positioned in the bore 226 of the housing212, the head portion 218 of the bone screw 214 may be inserted into thecavity 244 of the retainer 242 through the lower opening 238 from thelower end 230 in a bottom loaded manner. The diameter of the headportion 218 of the bone screw 214 may be less than the diameter of thelower opening 238 at the annular rim 256 to allow the head portion 218to pass therethrough. As shown in FIG. 3B, the head portion 218 of thebone screw 214, or another structure, may apply a force against theretainer 242 opposing and overcoming the biasing force of the wavewasher 252 which urges the retainer 242 to the second position in whichthe retainer 242 is moved toward the upper end 228 of the housing 212along the longitudinal axis of the bore 226. Now positioned in anenlarged diameter portion 236 of the bore 226 and radially unconstrainedby the interior surface of the bore 226 and/or the annular rim 256 atthe lower opening 238 of the housing 212, the flexibility of theretainer 242 allows the retainer 242 to be radially expanded. Forexample, the plurality of tabs 246 of the retainer 242 may be deflectedradially outward in order to allow the head portion 218 of the bonescrew 214 to pass into the cavity 244 of the retainer 242. The presenceof the annular space 254 allows the retainer 242 to radially expand toaccommodate insertion of the head portion 218 into the cavity 244.

Once the head portion 218 of the bone screw 214 is positioned in thecavity 244, the applied force to the retainer 242 may be removed,allowing the biasing force of the wave washer 252 or other biasing meansto move the retainer 242 back to the first position toward the lower end230 of the housing 212 and into engagement with the reduced diameterannular portion of the housing 212 to prevent further radial expansionor splaying of the retainer 242. In some instances, the retainer 242 mayinclude a lower beveled surface which contacts the annular rim 256 ofthe housing 212 to urge the tabs 246 of the retainer 242 radially inwardand/or prevent radial splaying to secure the head portion 218 of thebone screw 214 in the cavity 244 of the retainer 242.

When an elongate member 240 is secured in the channel 220 of the housing212, a clamping force may be exerted against the head portion 218 of thebone screw 214. However, because the lower opening of the retainer 242when at the first position has a diameter less than the diameter of thehead portion 218 of the bone screw 214, the head portion 218 isprevented from being removed from the cavity 244 of the retainer 242since the annular rim 256 of the housing 212 resists radial expansion ofthe lower opening of the retainer 242 when pressed thereagainst.

Another exemplary embodiment of a vertebral anchor 310, shown as apolyaxial pedicle screw, is illustrated in FIG. 4. The vertebral anchor310 may include several components. For example, the vertebral anchor310 may include a housing 312 pivotably coupled to a bone screw 314. Thebone screw 314 may include a shaft portion 316, which may in someinstances be threaded, extending from a head portion 318, which may insome instances be spherically shaped. The shaft 316 may be configured tobe installed into a bony region of a vertebra of the spinal column. Forexample, the shaft 316 may be installed into a pedicle of a vertebra, orother region of a vertebra. The bone screw 314 may be pivotable relativeto the housing 312 such that the longitudinal axis of the bone screw 314is positioned at one of multiple angular orientations relative to thelongitudinal axis of the housing 312.

The housing 312 may include a channel 320, such as a U-shaped channelextending from one side of the housing 312 to an opposite second side ofthe housing 312. The channel 320 may be defined between opposing firstand second legs 322, 324 of the housing 312. The housing 312 may alsoinclude a bore 326 extending through the housing 312 along alongitudinal axis from the upper end 328 to the lower end 330 of thehousing 312 which intersects the channel 320.

The housing 312 of the vertebral anchor 310 may be configured to receivean elongate member 340 of a vertebral stabilization system, such as arigid or flexible fixation element, including a spinal rod or flexiblecord, therein. For example, the channel 320 may be open to the upper end328 of the housing 312 such that the elongate member 340 may bepositioned in the channel 320 in a top-loaded fashion in which theelongate member 340 is moved into the channel 320 of the housing 312 ina direction generally perpendicular to the longitudinal axis of thechannel 320 of the housing 312.

The vertebral anchor 310 may also include a securing element, such as athreaded fastener (not shown) configured to rotatably engage the housing312 to secure a portion of the elongate member 340 in the channel 320.For example, the threaded fastener may include threads which mate with athreaded portion 332 formed in the legs 322, 324 of the housing 312. Inother embodiments, the fastener may include one or more flanges, camsurfaces, or other engagement features that engage with one or morechannels, grooves, surfaces, or other engagement features of the housing312 through rotation of the fastener. The fastener may be rotatablyengaged between the spaced apart legs 322, 324 of the housing 312 whichdefine the channel 320 therebetween.

The vertebral anchor 310 may also include one or more components forcoupling the housing 312 to the head portion 318 of the bone screw 314.For instance, the vertebral anchor 310 may include a retainer 342positionable in the bore 326 of the housing 312 which includes a cavity344 for receiving the head portion 318 of the bone screw 314 therein. Insome instances, the cavity 344 may be a spherically concave cavitycomplementing the spherical shape of the head portion 318 of the bonescrew 314. The retainer 342 may be formed of a resilient material, suchas a pliable polymeric material or a malleable metallic material,providing the retainer 342 a desired amount of flexibility. The retainer342 may also include a plurality of alternating tabs 346 and slots 348spaced around a periphery of the retainer 342 enhancing the flexibilityof the retainer 342. For example, a radially inward force may be exertedon the tabs 346 to deflect the tabs 346 radially inward to radiallycompress the retainer 342, whereas a radially outward force may beexerted on the tabs 346 to deflect or splay the tabs 346 radiallyoutward to radially enlarge the lower opening into the cavity 344 of theretainer 342.

The vertebral anchor 310 may further include a resilient spring meansbiasing the retainer 342 toward the lower end 330 of the housing 312. Asshown in FIG. 4, the resilient spring means may an integral portion ofthe retainer 342. For instance, the retainer 342 may be a portion of theretainer 342 including one or more circumferential or helical slots 352formed therein. Circumferential slots may extend less than 360° aroundthe circumference, whereas a helical slot may extend less than, greaterthan or equal to 360° around the retainer 342. The slots 352 may beinterposed between an annular upper portion 350 and the lower portionincluding the tabs 346 and defining the cavity 344. Thus, the retainer342 may be a monolithic structure including the annular upper portion350, the intermediate portion including the slots 352 and the lowerportion including the tabs 346. However, in other embodiments, theresilient spring means may be a wave washer, a helical spring,elastomeric member, or another structure configured to urge the retainer342 toward the lower end 330 of the housing 312.

The circumferential or helical slots 352 formed in the retainer 342 mayallow the intermediate portion of the retainer 342 to be resilientlycompressed between the annular upper portion 350 and the lower portionof the retainer 342. For example, when an axially compressive force isapplied to the retainer 342, the axial length of the retainer 342 may bereduced as the width of the slots 352 is reduced. When the appliedcompressive force is removed or reduced, the axial length of theretainer 342 may be increased. Thus, the slots 352 formed around theintermediate portion may provide a pseudo-spring.

The arrangement of components for coupling the housing 312 to the headportion 318 of the bone screw 314 is further illustrated in FIG. 4A. Asshown in FIG. 4A, the retainer 342 may be positioned in an enlargedportion 336 of the bore 326, surrounding the head portion 318 of thebone screw 314. The upper portion 350 of the retainer 342 may directlyengage the elongate member 340.

The retainer 342 may be movable in the bore 326 of the housing 312 alongthe longitudinal axis of the bore 326 between a first position in whichthe retainer 342 is closer to the lower end 330 of the housing 312 and asecond position in which the retainer 342 is closer to the upper end 328of the housing 312. The resilient nature of the compressed intermediateportion having the slots 352, or other resilient biasing means, may biasthe retainer 342 toward the first position until a sufficient force isapplied to the retainer 342 to overcome the biasing force of theintermediate portion having the slots 352 and moves the retainer 342 tothe second position.

The retainer 342 may have an outermost diameter which is greater thanthe diameter of the lower opening 338 of the bore 326 extending throughthe housing 312, yet the outermost diameter of the retainer 342 may beless than an enlarged portion 336 of the bore 326 in which the retainer342 is positioned, providing an annular space 354 between the outercircumferential surface of the retainer 342 and the circumferentialsurface of the bore 326. In some instances, the housing 312 may includean annular rim 356 defining the lower opening 338, in which the diameterof the lower opening 338 at the annular rim 356 is less than a diameterof the enlarged portion 336 of the bore 326 of the housing 312 towardthe upper end 328 of the housing 312 from the annular rim 356. When inthe first position, the forces generated by the compressed intermediateportion having the slots 352 may push the retainer 342 against theannular rim 356, preventing the retainer 342 from radially expanding.

During assembly of the vertebral anchor 310, the retainer 342 may beinserted into the lower opening 338 of the housing 312. For example, theplurality of alternating tabs 346 and slots 348 formed around thecircumference of the retainer 342 may provide the retainer 342 withsufficient flexibility to be urged through the lower opening 338 fromthe lower end 330 of the housing 312 by radially compressing theretainer 342.

With the retainer 342 positioned in the bore 326 of the housing 312, thehead portion 318 of the bone screw 314 may be inserted into the cavity344 of the retainer 342 through the lower opening 338 from the lower end330 in a bottom loaded manner. The diameter of the head portion 318 ofthe bone screw 314 may be less than the diameter of the lower opening338 at the annular rim 356 to allow the head portion 318 to passtherethrough. As shown in FIG. 4B, the head portion 318 of the bonescrew 314, or another structure, may apply a force against the retainer342 opposing and overcoming the biasing force of the intermediateportion having the slots 252 which urges the retainer 342 to the secondposition in which the retainer 342 is moved toward the upper end 328 ofthe housing 312 along the longitudinal axis of the bore 326. Nowpositioned in an enlarged diameter portion 336 of the bore 326 andradially unconstrained by the interior surface of the bore 326 and/orthe annular rim 356 at the lower opening 338 of the housing 312, theflexibility of the retainer 342 allows the retainer 342 to be radiallyexpanded. For example, the plurality of tabs 346 of the retainer 342 maybe deflected radially outward in order to allow the head portion 318 ofthe bone screw 314 to pass into the cavity 344 of the retainer 342. Thepresence of the annular space 354 allows the retainer 342 to radiallyexpand to accommodate insertion of the head portion 318 into the cavity344.

Once the head portion 318 of the bone screw 314 is positioned in thecavity 344, the applied force to the retainer 342 may be removed,allowing the biasing force of the intermediate portion having the slots252 or other biasing means to move the retainer 342 back to the firstposition toward the lower end 330 of the housing 312 and into engagementwith the reduced diameter annular portion of the housing 312 to preventfurther radial expansion or splaying of the retainer 342. In someinstances, the retainer 342 may include a lower beveled surface whichcontacts the annular rim 356 of the housing 312 to urge the tabs 346 ofthe retainer 342 radially inward and/or prevent radial splaying tosecure the head portion 318 of the bone screw 314 in the cavity 344 ofthe retainer 342.

When an elongate member 340 is secured in the channel 320 of the housing312, a clamping force may be exerted against the head portion 318 of thebone screw 314. However, because the lower opening of the retainer 342when at the first position has a diameter less than the diameter of thehead portion 318 of the bone screw 314, the head portion 318 isprevented from being removed from the cavity 344 of the retainer 342since the annular rim 356 of the housing 312 resists radial expansion ofthe lower opening of the retainer 342 when pressed thereagainst.

Yet another exemplary embodiment of a vertebral anchor 410, shown as apolyaxial pedicle screw, is illustrated in FIG. 5. The vertebral anchor410 may include several components. For example, the vertebral anchor410 may include a housing 412 pivotably coupled to a bone screw 414. Thebone screw 414 may include a shaft portion 416, which may in someinstances be threaded, extending from a head portion 418, which may insome instances be spherically shaped. The shaft 416 may be configured tobe installed into a bony region of a vertebra of the spinal column. Forexample, the shaft 416 may be installed into a pedicle of a vertebra, orother region of a vertebra. The bone screw 414 may be pivotable relativeto the housing 412 such that the longitudinal axis of the bone screw 414is positioned at one of multiple angular orientations relative to thelongitudinal axis of the housing 412.

The housing 412 may include a channel 420, such as a U-shaped channelextending from one side of the housing 412 to an opposite second side ofthe housing 412. The channel 420 may be defined between opposing firstand second legs 422, 424 of the housing 412. The housing 412 may alsoinclude a bore 426 extending through the housing 412 along alongitudinal axis from the upper end 428 to the lower end 430 of thehousing 412 which intersects the channel 420.

The housing 412 of the vertebral anchor 410 may be configured to receivean elongate member 440 of a vertebral stabilization system, such as arigid or flexible fixation element, including a spinal rod or flexiblecord, therein. For example, the channel 420 may be open to the upper end428 of the housing 412 such that the elongate member 440 may bepositioned in the channel 420 in a top-loaded fashion in which theelongate member 440 is moved into the channel 420 of the housing 412 ina direction generally perpendicular to the longitudinal axis of thechannel 420 of the housing 412.

The vertebral anchor 410 may also include a securing element, such as athreaded fastener (not shown) configured to rotatably engage the housing412 to secure a portion of the elongate member 440 in the channel 420.For example, the threaded fastener may include threads which mate with athreaded portion 432 formed in the legs 422, 424 of the housing 412. Inother embodiments, the fastener may include one or more flanges, camsurfaces, or other engagement features that engage with one or morechannels, grooves, surfaces, or other engagement features of the housing412 through rotation of the fastener. The fastener may be rotatablyengaged between the spaced apart legs 422, 424 of the housing 412 whichdefine the channel 420 therebetween.

The vertebral anchor 410 may also include one or more components forcoupling the housing 412 to the head portion 418 of the bone screw 414.For instance, a lower portion of the housing 412 may include a cavity444 for receiving the head portion 418 of the bone screw 414 therein. Insome instances, the cavity 444 may be a spherically concave cavitycomplementing the spherical shape of the head portion 418 of the bonescrew 414. The lower portion of the housing 412 may be formed of aresilient material, such as a pliable polymeric material or a malleablemetallic material, providing the lower portion of the housing 412 adesired amount of flexibility. The lower portion of the housing 412 mayinclude a plurality of alternating tabs 446 and slots 448 spaced arounda periphery of the lower portion of the housing 412 enhancing theflexibility of the lower portion of the housing 412. For example, aradially inward force may be exerted on the tabs 446 to deflect the tabs446 radially inward, whereas a radially outward force may be exerted onthe tabs 446 to deflect or splay the tabs 446 radially outward toradially enlarge the lower opening 438 into the cavity 444 of the lowerportion of the housing 412.

A collar 470 may be provided which may be positioned circumferentiallyaround the tabs 446 of the lower portion of the housing 412 to preventradial outward deflection or splaying of the tabs 446 when the housing412 is coupled to the bone screw 414. In some instances, the collar 470may include opposing flat or planar side surfaces to facilitatemanipulation of the collar 470.

The vertebral anchor 410 may also include a spacer 450 positioned in thebore 426 having a first portion which is configured to engage anelongate stabilization member 440 disposed in the channel 420 of thehousing 412 and a second portion which is configured to engage the headportion 418 of the bone screw 414.

The vertebral anchor 410 may further include a resilient spring meansbiasing the spacer 450 toward the lower end 430 of the housing 412 andinto engagement with the head portion 418 of the bone screw 414. Asshown in FIG. 5, the resilient spring means may be a wave washer 452,however, in other instances the resilient spring means may be a helicalspring, elastomeric member, an integral portion of the spacer 450, oranother structure configured to urge the spacer 450 toward the lower end430 of the housing 412.

The arrangement of components for coupling the housing 412 to the headportion 418 of the bone screw 414 is further illustrated in FIG. 5A. Asshown in FIG. 5A, the head portion 418 of the bone screw 414 may bepositioned in the cavity 444, surrounded by the tabs 446 of the housing412. The spacer 450 may be positioned between the head portion 418 ofthe bone screw 414 and the elongate member 440, with a portion of thespacer 450 extending through the wave washer 452. The wave washer 452may be positioned between the spacer 450 and the housing 412 andcompressed between an annular rim of the housing 412 facing the lowerend 430 of the housing 412 and an annular surface of an enlargeddiameter portion of the spacer 450 facing the upper end 428 of thehousing 412. The collar 470 may be positioned circumferentially aroundthe tabs 446 of the lower portion of the housing 412, preventing radialoutward deflection or splaying of the tabs 446.

During assembly of the vertebral anchor 10, the head portion 418 of thebone screw 412 may be passed into the cavity 444 of the lower portion ofthe housing 412 from the lower end 430 of the housing 412 by radiallydeflecting or splaying the plurality of tabs 446 radially outward from afirst, equilibrium position to a second position in which the diameterof the annular rim 456 is greater than or equal to the diameter of thehead portion 418, as shown in FIG. 5B.

Once the head portion 418 of the bone screw 414 is positioned in thecavity 444 above 5 the annular rim 456, the tabs 446 will move backtoward the first, equilibrium position with the annular rim 456surrounding a portion of the head portion 418 below the greatest extentof the head portion 418. The collar 470 may then be positionedcircumferentially around the tabs 446 of the lower portion of thehousing 412, preventing further radial outward deflection or splaying ofthe tabs 446. In some instances, the collar 470 may include a beveledsurface 472 proximate the upper opening of the collar 470 whichfacilitates advancing the collar 470 over the annular rim 447 of thetabs 446.

When an elongate member 440 is secured in the channel 420 of the housing412, a clamping force may be exerted against the head portion 418 of thebone screw 414. However, the annular rim 456 of the housing 414, whichhas a diameter less than the diameter of the head portion 418 of thebone screw 414 when at the first position, prevents the head portion 418from being removed from the cavity 444 since the collar 470 preventsradial slaying of the tabs 446.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departure in form anddetail may be made without departing from the scope and spirit of thepresent invention as described in the appended claims.

What is claimed is:
 1. A polyaxial bone anchor comprising: an upperhousing component comprising: an upper end; a lower end; a boreextending through the upper housing component from the upper end to thelower end along a longitudinal axis; and a channel configured forreceiving an elongate stabilization member therethrough which extendsfrom a first side surface of the upper housing component to a secondside surface of the upper housing component opposite the first sidesurface transverse to the bore; a lower housing component attached tothe lower end of the upper housing component so that at least a portionof a radially outer surface of the lower housing component is exposed toan exterior of the polyaxial bone anchor, wherein the lower housingcomponent is rotatable relative to the upper housing component about thelongitudinal axis; a spacer positioned within the bore proximate thelower end of the upper housing component at least partially within thelower housing component such that the upper housing component, the lowerhousing component and the spacer are at least partially concentric; aring positioned concentrically between the spacer and the upper housingcomponent and that axially engages the upper housing component; and abone screw including a head portion and a shank extending from the headportion, the head portion of the bone screw positionable in the bore toengage the spacer without engaging the ring and with the shank extendingfrom the lower end of the upper housing component and the head portionengaging the spacer.
 2. The polyaxial bone anchor of claim 1, whereinthe lower housing component is rotatable three-hundred-sixty degreesabout the longitudinal axis when an elongate stabilization member issecured in the channel.
 3. The polyaxial bone anchor of claim 1, whereinthe lower housing component includes an inner diameter passage that islarger than a width of the head portion of the bone screw and into whichthe bore of the upper housing component extends.
 4. The polyaxial boneanchor of claim 1, wherein an inner surface of the lower housingcomponent circumscribes on outer surface of the spacer at a locationalong the longitudinal axis.
 5. The polyaxial bone anchor of claim 4,wherein the lower housing component circumscribes the lower end of theupper housing component.
 6. The polyaxial bone anchor of claim 4,wherein the non component is at least partially within the bore.
 7. Thepolyaxial bone anchor of claim 1, wherein the lower housing componentinteracts with the upper housing component to restrain axial movement ofthe bone screw.
 8. The polyaxial bone anchor of claim 1, furthercomprising one or more deflectable members positioned over the headportion of the bone screw to inhibit the bone screw from passing throughthe lower housing component.
 9. The polyaxial bone anchor of claim 8,wherein the lower housing component restrains outward deflection of theone or more deflectable members.
 10. The polyaxial bone anchor of claim8, wherein the one or more deflectable members extend from the spacer.11. The polyaxial bone anchor of claim 8, wherein the one or moredeflectable members comprise the lower end of the upper component. 12.The polyaxial bone anchor of claim 1, wherein the spacer is configuredto be positioned within the bore to rest on the head portion of the bonescrew such that the elongate stabilization rod can rest on the spacer,the spacer having a planer upper surface.
 13. The polyaxial bone anchorof claim 1, wherein the ring is located within the bore of the upperhousing component and is further concentric with the lower housingcomponent.
 14. The polyaxial bone anchor of claim 13, wherein the ringcomprises a biasing element.
 15. A polyaxial bone anchor comprising: anupper housing component comprising: an upper end; a lower end; a boreextending through the upper housing component from the upper end to thelower end along a longitudinal axis; and a channel configured forreceiving an elongate stabilization member therethrough which extendsfrom a first side surface of the upper housing component to a secondside surface of the upper housing component opposite the first sidesurface transverse to the bore; a lower housing component separate fromand coupled to the lower end of the upper housing component such that anupward-facing surface of the lower housing component faces adownward-facing feature of the upper housing component; a spacerpositioned within the bore proximate the lower end of the upper housingcomponent at least partially within the lower housing component, thespacer including an upward-facing surface that faces the downward-facingfeature of the upper housing component; a ring positioned against thedownward-facing feature of the upper housing component and the lowerhousing component and radially between the upper housing component andthe spacer; a bone screw including a head portion and a shank extendingfrom the head portion, the head portion of the bone screw positionablein the bore to engage the spacer and with the shank extending from thelower end of the upper housing component and the head portion engagingthe spacer; and a plurality of deflectable members coupled to the headportion of the bone screw to inhibit axial displacement of the bonescrew out of the bore.
 16. The polyaxial bone anchor of claim 15,wherein the lower housing component circumscribes the plurality ofdeflectable members and restrains the plurality of deflectable membersfrom radially expanding.
 17. The polyaxial bone anchor of claim 16,wherein the plurality of deflectable members extend from the upperhousing component.
 18. The polyaxial bone anchor of claim 17 wherein thelower housing component is configured to hold the plurality ofdeflectable members in a relaxed state to prevent radial outwarddeflection of the plurality of deflectable members.
 19. The polyaxialbone anchor of claim 15, wherein the plurality of deflectable membersextend from the spacer.
 20. The polyaxial bone anchor of claim 19,wherein the plurality of deflectable members are coupled to the spacervia the lower housing component.
 21. The polyaxial bone anchor of claim15, wherein the lower housing component is rotatable relative to theupper housing component about the longitudinal axis.
 22. The polyaxialbone anchor of claim 15, wherein the has a larger diameter than thespacer.
 23. The polyaxial bone anchor of claim 22, wherein the ring ispositioned between the upper housing component and the lower housingcomponent to bias the lower housing component away from the upperhousing component.
 24. The polyaxial bone anchor of claim 22, whereinthe upper housing component, the spacer and the ring are at leastpartially concentric.
 25. The polyaxial bone anchor of claim 15, whereinthe spacer is positioned within the bore to extend into both the upperhousing component and the lower housing component.
 26. The poly axialbone anchor of claim 15, wherein the spacer is configured to seat on thehead portion of the bone screw and engage the upper housing component.